Volume 3: General-Purpose and System Instructions - Stanford ...

24594 Rev. 3.10 February 2005 AMD64 TechnologyA.2.4 ModRMExtensions to One-Byte and Two-ByteOpcodesThe ModRM byte, which immediately follows the last opcodebyte, is used in certain instruction encodings to provideadditional opcode bits with which to define the function of theinstruction. ModRM bytes have three fields—mod, reg, and r/m,as shown in Figure A-1.Bits:7 6 5 4 3 2 1 0mod reg r/m ModRM513-325.epsFigure A-1.ModRM-Byte FieldsIn most cases, the reg field (bits 5–3) provides the additionalbits with which to extend the encodings of the first one or twoopcode bytes. In the case of the x87 floating-point instructions,the entire ModRM byte is used to extend the opcode encodings.Table A-6 on page 388 shows how the ModRM reg field is used toextend the range of one-byte and two-byte opcodes. The opcoderanges are organized into groups of opcode extensions. Thegroup number is shown in the left-most column of Table A-6.These groups are referenced in the opcodes shown in Table A-1on page 378 through Table A-4 on page 383. An entry of “n.a.”in the Prefix column means that prefixes are not applicable tothe opcodes in that row. Prefixes only apply to certain 128-bitmedia, 64-bit media, and a few other instructions introducedwith the SSE or SSE2 technologies.The /0 through /7 notation for the ModRM reg field (bits 5–3)means that the three-bit field contains a value from zero (binary000) to 7 (binary 111).Appendix A: Opcode and Operand Encodings 387